Due to increased emission standards, nowadays vehicles typically include a fuel vapour recovery system. Such a fuel vapour recovery system includes a canister for receiving fuel vapours generated in the fuel tank. A fuel vapour absorbent material located in the canister retains the fuel vapour when displaced from the fuel tank, e.g. during refueling. During operation of the engine, the fuel vapour contained in the canister may be purged by drawing fresh air through the canister. In fuel vapour recovery systems of the prior art, typically there is provided a vapour vent valve between the tank and an inlet of the canister for being capable of blocking the entrance of vapour from the fuel tank in the canister. Further, there may be provided a canister vent valve between an air vent and an outlet of the canister. For example, during filling or at elevated temperatures, the vapour vent valve and the canister vent valve are open, so that a fuel vapour can flow from the fuel tank into the canister, and fresh air can flow out in the atmosphere through the canister vent valve, allowing the pressure in the fuel tank to be reduced. During normal engine operation, the vapour vent valve may be closed while the canister vent valve is open to allow the flow of air into the outlet of the canister, through the canister medium and through a canister purge valve allowing the fuel vapour stored in the canister to be delivered to the engine.
In such prior art systems it is difficult to control the amount of vapours that is being removed from the tank during refueling and/or during purging. Prior art systems may be pressure-based, wherein the refueling is controlled on the basis of pressure measurements. However, if the pressure sensor fails, a good refueling cannot be obtained. Further, prior art systems are difficult to calibrate in view of hard tooling requirements. Generally speaking the usable volume of a fuel tank can only be predicted to an accuracy of around 10% before actual tanks are produced. Since the associated float valves that typically control the shutoff have a similar lead time to construct as the tank, they are generally made based on a prediction of what the fluid height in the tank would be at the rated capacity. Since this fluid height often varies, a certain amount of physical adjustment to the shutoff height of the valves is required. Since these are hard tooled components, it often means making small modifications to the injection moulds and re-running parts. This is a process that can take as much as several weeks to carry out. In addition the valves are usually fusion welded to the tank shell and therefore new prototype tanks must be allocated to allow for welding the modified valves to.